1. Field of the Invention
The present invention relates to a semiconductor device and to a method of manufacturing the same. More particularly, the present invention relates to a mask having a dummy pattern for correcting for an optical proximity effect (OPE) occurring when forming fine patterns, and to a method of manufacturing such a mask.
2. Description of the Related Art
One of the problems in forming fine patterns in a semiconductor device having a design rule of less than 180 nm is in securing a focus margin in a photolithography process. In particular, a semiconductor device may have patterns of various line widths and pitches, that is, a region having dense patterns and a region having an isolated pattern may be formed on the same chip. In the case of manufacturing such a semiconductor device, the focus margin for simultaneously forming dense device patterns and an isolated device pattern on a wafer is rather limited due to an optical proximity effect (OPE). The reason for this is that dense patterns and an isolated pattern have different diffraction patterns.
FIG. 1A illustrates a diffraction pattern formed on a projection lens by exposing the projection lens to light transmitted through dense patterns on a mask. In this case, dense patterns having line widths of 0.11 μm and a pitch of 0.23 μm are exposed to a KrF light source. The x-axis of the graph denotes the sine of the diffraction angle α, and the y-axis denotes the amplitude of the diffracted light. As shown in FIG. 1A, the diffracted light of the dense pattern results in a discontinuous distribution of peaks such as the zero-order maximum, the ± first-order maxima, etc.
On the other hand, FIG. 1B illustrates a diffraction pattern formed on a projection lens by exposing the projection lens to light transmitted through an isolated pattern on a mask. In this case, an isolated pattern having a line width of 0.2 μm is exposed to a KrF light source. As shown in FIG. 1B, only the zero-order maximum is formed.
If the light having different diffraction patterns is simultaneously projected onto a wafer through the same projection lens to form a device pattern, a difference between “isolated” and “dense” patterns would occur.
To obviate such a problem, a method using a scattering bar has been suggested. A scattering bar is a dummy pattern, finer than a main pattern, and formed on a mask substrate at both sides of isolated patterns. However, the smaller the design rule, that is, the smaller the line width of the patterns, the more difficult it becomes to form a scattering bar having a fine enough line width. Also, manufacturing a mask having a scattering bar is quite difficult because of the strict process conditions required.
Meanwhile, an optimal off-axis exposure method, known as having a high degree of resolution, could be used to form a dense pattern. However, this method only exacerbates the difference between the “isolated” and “dense” patterns. More specifically, in the off-axis exposure method used to produce a dense pattern, the optical axis is not perpendicular to the surface of a wafer, that is, the center of the optical axis is in a region of the dense pattern. As a result, the depth of focus of light on the isolated pattern is relatively small, and thus, the total depth of focus is small.
Depth of focus dictates the line width, i.e., the fineness of the pattern. That is, a pattern having a line width within the depth of focus can be precisely formed. Thus, the larger the depth of focus is, the easier it is to precisely form a pattern, and the smaller the depth of focus is, the more difficult it is to precisely form a pattern.